[Bug] Unhandled database schema mismatch causes prompt_async silent failure and client request timeout

Summary

When opencode serve (CLI) and OpenCode Desktop share the same database (~/.local/share/opencode/opencode.db), upgrading Desktop ahead of CLI creates a database schema mismatch (e.g. newer migrations like simplify_session_context_epoch removing columns).

Steps to Reproduce

1. OpenCode Desktop runs on version 1.18.x and applies new database migrations to ~/.local/share/opencode/opencode.db.
2. An older CLI binary (1.17.x) is run in headless server mode (opencode serve).
3. A client connects, creates a session, and sends a prompt/message (/session/:id/prompt_async).

Actual Behavior

1. opencode serve crashes internally on prompt_async with:
   prompt_async failed cause="Cause([Die(SQLiteError: no such column: replacement_seq)])"
2. No error event or failure response is returned/streamed to the client/frontend.
3. The client hangs waiting for SSE stream until it eventually times out.

Expected Behavior

1. OpenCode should validate database schema compatibility on startup/query and log a user-friendly error (e.g., "Database schema is newer than binary version, please update opencode").
2. If prompt_async or LLM orchestration dies with an unhandled exception, an error event should be dispatched to the client immediately instead of hanging indefinitely.

Additional Context

• OS: Ubuntu 25.10 / Linux
• CLI Version: 1.17.7
• Desktop Version: 1.18.23